Why segmentation fault

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Article Contributed By :. Rajendra Uppal Rajendra Uppal Just a memory dump when something goes wrong! Typically called by attempting to dereference a null pointer, so a segmentation fault is often analogous to a Java NullPointerException.

Segmentation comes from Memory Segmentation. You're accessing a segment of memory that does not belong to you. Add a comment. Active Oldest Votes. Improve this answer. Community Bot 1 1 1 silver badge. No warnings on compile. No segfault. The code can run fine on a production system for years, you alter another part of the code, change compiler or something else and BOOOOOM!

Sorry for the bump but just a side note Not least because there are systems out there that operate without memory protection, thus cannot tell whether a piece of memory actually "belongs to you", and thus don't know segfaults, only undefined behaviour ChrisHuang-Leaver, normally when you get out of scope, the compiler has to recover some stack space to free the unused stack space, but this doesn't happen always with gcc being one of this compilers.

Also, the allocated stack space is normally reused again, so I have heard of no operating systems that return unused stack pages to the system, making that space subject for a SIGSEGV , so I won't expect such a signal from mangling with the stack.

Show 4 more comments. And all of this with respect to virtual memory systems. This is what I had on mind in "And things like shmat ; - these are what I count as 'indirect' access. In a virtual memory operating system there's no way normally, so please, operating system implementors, don't flame me for this for a process to access another process virtual memory, not being some kind of memory attach system call that allows you to access.

Virtual memory addresses normally mean different things depending on the process being considered. Ignacio Vazquez-Abrams Ignacio Vazquez-Abrams k gold badges silver badges bronze badges. These kinds of errors are usually caused by pointers that are Used before being properly initialised Used after the memory they point to has been realloced or deleted. Used in an indexed array where the index is outside of the array bounds. Component 10 Component 10 9, 5 5 gold badges 43 43 silver badges 59 59 bronze badges.

According to Wikipedia : A segmentation fault occurs when a program attempts to access a memory location that it is not allowed to access, or attempts to access a memory location in a way that is not allowed for example, attempting to write to a read-only location, or to overwrite part of the operating system.

Fabio says Reinstate Monica 4, 7 7 gold badges 38 38 silver badges 52 52 bronze badges. Orhan Cinar Orhan Cinar 8, 2 2 gold badges 34 34 silver badges 48 48 bronze badges. The solution in this case, change the RAM. Alejo Bernardin Alejo Bernardin 11 11 silver badges 20 20 bronze badges.

A quick-and-dirty test for faulty RAM is to run your crashing program over and over again in a loop. If the program has no internal nondeterminism—that is, it always produces the same output for the same input, or at least it's supposed to—but, for some particular input, it crashes sometimes , not always but not never either: then you should start worrying about bad RAM.

A stack overflow. If you can't find the problem any other way, you might try a debugger. For example, you could use GNU's well-known debugger GDB to view the backtrace of a core file dumped by your program; whenever programs segfault, they usually dump the content of their section of the memory at the time of the crash into a core file.

Start your debugger with the command gdb core , and then use the backtrace command to see where the program was when it crashed. This simple trick will allow you to focus on that part of the code. If using backtrace on the core g file doesn't find the problem, you might have to run the program under debugger control, and then step through the code one function, or one source code line, at a time. To do this, you will need to compile your code without optimization, and with the -g flag, so information about source code lines will be embedded in the executable file.

This is document aqsj in the Knowledge Base. Last modified on If you are running a program that you downloaded from the Internet and aren't familiar with the source code, you may be out of luck: All you can do is submit a bug report and hope for a fix. Just to be sure, make sure you are running the most up-to-date version of the software.

In addition, check the bug report site to see if the bug has already been reported, and if any temporary workarounds or patches are available to download. The only way to avoid segfaults completely in written programs is to be careful with memory allocations and deletions and track down errors as they occur.

Finding the exact source of the error can be tricky, especially since it may not show up every time you run the program. In addition, the actual bug may not be anywhere near where the program crashes, since a faulty memory allocation may only crash the program when it is first accessed.